Cell Signaling and Signal Transduction

Overview of Cell Signaling

• Cells respond to chemical messages (ligands) from other cells or the environment.
• Cell signaling begins when a ligand is received by a target cell.

Types of Ligands

• Ligands can include:
  • Peptides
  • Proteins
  • Steroids
  • Small chemicals

Receptors

• Ligands bind to specific cell structures called receptors.
• Types of Receptors:
  • Some are located on the nuclear membrane.
  • Most are embedded in the cell membrane.

Key Domains of Receptors

1. Ligand Binding Domain
   • Extracellular; specific chemical structure for the ligand.
2. Transmembrane Domain
   • Anchors the protein in the plasma membrane.
   • Some have channels for transport (e.g., gated ion channels).
3. Intracellular Domain
   • Undergoes conformational changes upon ligand binding, altering its function.

G-Protein-Coupled Receptors (GPCRs)

• Important membrane receptors in eukaryotes.
• Have an intracellular domain associated with G-proteins.
• Ligand binding causes G-proteins to dissociate, beginning the signal transduction pathway.

Signal Transduction Pathway

• Signal Transduction: Series of chemical events from ligand binding to cellular response.
• Involves activation of other molecules (channels or enzymes) by dissociated G-proteins.
• Activated enzymes produce Second Messengers (e.g., cyclic AMP) for signal amplification.

Phosphorylation Cascade

• Phosphorylation: Addition of phosphate groups, causing shape changes in proteins.
• Activated by second messengers like cyclic AMP to activate Protein Kinases.
• Kinases transfer phosphate groups in a relay-like manner, activating/deactivating proteins.

Target Proteins

• Target proteins include:
  • Enzymes controlling metabolic processes.
  • Transcription factors regulating gene expression.
• The final response of a signal transduction pathway is determined by the target protein's function.

Importance of Signal Transduction

• Sequential process from ligand binding to cellular response.
• Disruption at any step can alter cellular response.